Tinned steel sheets referred to as “tinplate” have been widely used as surface-treated steel sheets for cans. In those tinned steel sheets, chromate coatings are formed on tin plating layers by chromating such that steel sheets are immersed in aqueous solutions containing a hexavalent chromium compound such as bichromic acid or are electrolyzed in the aqueous solutions. This is because the formation of the chromate coatings prevents the surface oxidation of the tin plating layers, which are likely to be oxidized during long-term storage, to suppress the deterioration of appearance (yellowing) and also prevents cohesive failure due to the growth of tin (Sn) oxide coatings to secure the adhesion (hereinafter simply referred to as “paint adhesion”) with organic resins such as paints in the case of painting the tinned steel sheets.
In light of recent environmental issues, efforts to restrict the use of Cr are being made in every field. For tinned steel sheets for cans, several chemical conversion techniques alternative to chromating have been proposed.
For example, Japanese Examined Patent Application Publication No. 55-24516 discloses a method for surface-treating a tinned steel sheet. In that method, a chemical conversion coating is formed such that the tinned steel sheet is subjected to direct-current electrolyzing in a phosphate solution using the tinned steel sheet as a cathode. Japanese Examined Patent Application Publication No. 58-41352 discloses a chemical conversion solution which contains phosphoric ions, tin ions, and one or more of a chlorate and a bromate and which has a pH of 3 to 6. Japanese Unexamined Patent Application Publication No. 49-28539 discloses a method for surface-treating tinplate. In that method, one or more of calcium phosphate, magnesium phosphate, and aluminum phosphate are applied to tinplate to form a coating with a thickness corresponding to 15 μg/cm2 or less. Japanese Unexamined Patent Application Publication No. 2005-29808 discloses a surface-treated steel sheet for containers. In the surface-treated steel sheet, an iron-nickel (Fe—Ni) diffusion layer, an Ni layer, an Ni—Sn alloy layer, and a non-alloyed Sn layer are arranged on a surface of a steel sheet in that order and a phosphoric acid coating having a mass per unit area of 1 to 100 mg/m2 in terms of phosphorus (P) is disposed on the non-alloyed Sn layer.
The chemical conversion coatings disclosed in JP '516, JP '352, JP '539 and JP '808 are less capable of suppressing the deterioration of appearance and reduction of paint adhesion due to the surface oxidation of tin plating layers when compared to conventional chromate coatings.
Japanese Unexamined Patent Application Publication No. 2007-239091 discloses a method for producing a tinned steel sheet. In that method, after a steel sheet is tinned, a chemical conversion coating is then formed such that the steel sheet is immersed in a chemical conversion solution containing tin ions and phosphoric ions or cathodically electrolyzed in the chemical conversion solution and is then heated to a temperature of 60° C. to 200° C. The chemical conversion coating can suppress the deterioration of appearance and the reduction of paint adhesion due to the surface oxidation of a tin plating layer equally to or better than conventional chromate coatings.
The method disclosed in JP '091 has the problem that a heating unit used subsequently to chemical conversion is necessary and therefore the cost of chemical conversion is high.
It could therefore be helpful to provide a tinned steel sheet which can suppress the deterioration of appearance and reduction of paint adhesion due to surface oxidation of a tin plating layer without using Cr and which can be subjected to chemical conversion at low cost and to provide a method for producing such a tinned steel sheet.